As new technologies employing bonded structures are developed, the use of Thermo Compression Bonding (TCB) is becoming more prevalent. TCB can for example be used for wire bonding, Flip Chip bonding or die stacking. It needs protruding contacts, e.g. stud bumps created from Au or Cu wires or Cu pillars with Sn based solder tips. These protruding contacts can be located on either the substrate or on another component of the bonded structure. Opposite to the protruding contacts flat contacts, preferably of the same material as the protruding contacts, serve as bonding partners and are called bonding landing pads. This material can be non-noble metals.
The TCB process realizes a final bond between these two components of the bonded structure by applying a pressure force profile and a heat profile on the two components to be bonded. Typically an underfill material is present between the two components during the TCB process.
In general there is a storage time between the manufacturing of the components to be bonded in a bonded structure, prior to them being bonded. It should be further noted that the assembly process itself and the storage is not necessary a non-oxidizing environment. These factors ultimately may lead to the oxidation of the non-noble metallic bonding landing pads. It is a well-known fact that oxidation of the non-noble metallic bonding landing pad often causes solder joint formation issues during the bonding process. This is especially true when the underfill material does not have enough fluxing capability to remove the non-noble metal oxides during the bonding process.
Coating or capping the metallic bonding landing pad with a thick (>0.5 um) Electro Less Deposition (ELD) of NiP, NiB with low (up to 5% by weight) Boron content, Au, Pd or stacked combinations thereof, are used to prevent oxidation of the metallic bond landing pad. An example can be found in U.S. Pat. No. 7,078,796 B2. These solutions require noble metals like Au or Pd having excellent non oxidizing properties, but which are expensive. These noble metals have excellent anti-oxidation properties, but are rare materials and expensive.